The present invention concerns generally the field of current generator circuits. More particularly, the present invention relates to a method for generating a substantially temperature independent current and a device allowing implementation of the same.
Current generator circuits, commonly known by the name of xe2x80x9ccurrent sourcesxe2x80x9d or xe2x80x9ccurrent sinksxe2x80x9d are important elements in the design of numerous electric and electronic circuits. FIG. 1 shows an example of a current generator circuit of the prior art globally designated by the reference numeral 10. This current generator circuit 10 constitutes a voltage controlled current generator circuit.
Current generator circuit 10 typically includes amplifying means formed of an operational amplifier or differential amplifier 11, a transistor 12 and a resistor 13. Operational amplifier 11 includes a positive input terminal (non inverting input) 11a at which is applied an input voltage designated Vin, a negative input terminal (inverting input) 11b and an output 11c. Amplifying means 11 supplies a voltage at its output 11c in response to a difference between the voltages applied respectively to its first and second input terminals 11a and 11b. 
Transistor 12 is formed in this example of an n-MOS field effect transistor whose gate 12c is connected to the output 11c of operational amplifier 11. Source 12a of transistor 12 is connected to negative input 11b of operational amplifier 11 and to a first terminal of resistor 13. The other terminal of resistor 13 is connected to a supply potential or reference potential Vss. This reference potential Vss is typically defined as the most negative potential of the circuit or the circuit""s earth at 0 volts. Another supply potential Vdd (not illustrated in FIG. 1) is also provided. Potentials Vss and Vdd constitute supply voltages for the circuit, and particularly for operational amplifier 11.
According to the current generator circuit of FIG. 1, a current designated 11 passes through the drain-source branch 12a-12b of MOS transistor 12. The analysis of this circuit is direct. Operational amplifier 11 modifies the voltage at its output 11c such that the voltage present at its negative input 11b is substantially equal to the voltage present at its positive input 11a, i.e. substantially equal to input voltage Vin. The voltage across the terminals of resistor 13 is thus substantially equal to input voltage Vin, such that current 11 passing through the drain-source branch of MOS transistor 12 is given by:                     I1        =                  Vin          R                                    (        1        )            
where R is the value of resistor 13. Generated current I1 is thus proportional to input voltage Vin applied at positive input 11a of the operational amplifier.
Current generator circuit 10 of FIG. 1 forms a xe2x80x9ccurrent sinkxe2x80x9d, i.e. a current I1 is drained from drain 12b of transistor 12 towards the most negative potential Vss. A modification of circuit 10 of FIG. 1 allows a current source to be formed. FIG. 2 illustrates a generator circuit designated 20 showing such a modification. Identical reference numerals are used to indicate those elements which have already been presented, i.e. operational amplifier 11, MOS transistor 12 and resistor 13.
In addition to the elements already mentioned, generator circuit 20 of FIG. 2 typically includes a current mirror 30 formed of first and second p-MOS field effect transistors respectively designated 31 and 32. Sources 31a and 32a of transistors 31 and 32 are connected to the most positive supply potential Vdd. Gate 31c and drain 31b of transistor 31 are connected together to drain 12b of transistor 12 and gate 32c of transistor 32 is connected to gate 31c of transistor 31.
Current mirror 30 thus operates so as to xe2x80x9ccopyxe2x80x9d current 11 and generate a current which is the image of current I1 in the drain-source branch of transistor 32. In accordance with what is typically known in the field, a proportionality factor can be introduced into the mirror by a suitable choice of the channel width to length ratios W/L of MOS transistors 31, 32 in order to multiply or divide current I1.
Circuit 20 of FIG. 2 may of course be further modified so that the current mirror includes other branches, for example a third MOS field effect transistor 33 as indicated in FIG. 2 in order to generate a third current 13.
One problem of the current generator circuits illustrated in FIGS. 1 and 2 lies in particular in the temperature dependence of the currents generated. Typically, a temperature stable voltage such as a reference bandgap voltage approximately equal to 1.2 volts is used as input voltage Vin. This reference bandgap voltage has a relative low temperature dependence of the order of 50 ppm/xc2x0C.
In order to make resistor 13, it is also sought to use a resistor whose temperature coefficient is relatively low. For design reasons, it is also sought to make resistor 13 in an integrated form and to avoid using a resistor external to the circuit. Various solutions exist in CMOS technology to design integrated resistors. It can however be noted that the temperature coefficients of these integrated resistors remains relatively high with respect to the temperature stability of a reference bandgap voltage. By way of example, an integrated resistor of the Rpoly type, i.e. an integrated resistor formed of a polysilicon layer, typically has a temperature coefficient of the order of +0.07%/xc2x0C., namely a temperature coefficient which remains substantially significant with respect to the stability of a reference bandgap voltage.
Those skilled in the art quickly note that there is no satisfactory way available, in CMOS technology, of making integrated resistors with sufficiently low temperature coefficients. With the aim of making a current generator circuit of the aforementioned type, the current generated by means of such a circuit will thus have a temperature dependence essentially due to the temperature dependence of the integrated resistor used.
A general object of the present invention is thus to propose a method for generating a substantially temperature independent current by means of a current generator circuit of the aforementioned type.
Another object of the present invention is to propose a device allowing the aforementioned method to be implemented, namely a current generator circuit overcoming the drawbacks encountered with the use of integrated resistors and arranged to generate a substantially temperature independent current.
A further object of the present invention is to propose a solution which involves only a few modifications to the current generator circuit and which consequently proves simple and inexpensive to manufacture with respect to the already existing solutions.
In order to answer these objects, the present invention first concerns a method for generating a substantially temperature independent current the features of which are listed in claim 1.
The present invention also concerns a current generator circuit the features of which are listed in claim 5.
The present invention relies on the observation by the inventor of the possibility of compensating for the temperature dependence of the current due to the resistor used by acting on the geometry of the differential pair of transistors of the operational amplifier used, in order to intentionally generate an offset voltage between the input terminals of the operational amplifier, this offset voltage being adjusted to have a temperature dependence compensating for the temperature dependence of the resistor used.
Indeed, the inventor was able to observe that by arranging the operational amplifier so as to create a geometric imbalance between the two transistors of the differential pair of said amplifier, an offset voltage between the input terminals of the amplifier was generated, this offset voltage having a substantially linear temperature dependence able to be adjusted by working with the geometry of the transistors of the differential pair, in particular by the bias of their dimensional channel width over length ratio W/L.
One advantage of the present invention lies in the simplicity of its implementation and in the low modification cost. Moreover, the offset voltage of the operational amplifier can be adjusted to have independently a positive or negative temperature coefficient according to whether one acts on one or the other of the transistors of the differential pair. It is thus possible to compensate for the temperature dependence of resistors having either a positive or a negative temperature coefficient.